<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        html {
            /* 浏览器的宽度除以设计图的宽度 */
            /* 1rem = 1px */
            /* font-size: calc(100vw / 750); */
            /* 1rem = 100px  */
            font-size: calc(100vw / 7.5);
        }

        .a {
            /* 在写宽高的时候  把px换算成rem    px / 100 */
            width: 5.26rem;
            height: .6rem;
            background-color: #f00;
            font-size: .28rem;
        }
    </style>
</head>

<body>

    <div class="a">哈哈</div>

</body>

</html>